1. Field of the Invention
The invention relates in general to magnetic resonance imaging (MRI), and in particular to devices for in vivo MRI.
2. Related Art
Evaluation of the anatomy of the urethra and the periurethral tissues is important for the diagnosis and treatment of the number of clinical conditions in men and women. Malignant conditions include cancers of the bladder, cancers of the cervix and female genital tract, and cancers of the prostate. Non-malignant conditions may include voiding disorders such as, for example, various types of incontinences in both men and women. In addition to adult conditions, anatomic diagnosis may assist in the evaluation of pediatric anomalies of the pelvis and urinary tract.
Evaluating the anatomy of the urethra and periurethral tissues may be useful in staging bladder cancers and in determining desirable methods of treatment. Cancers of the bladder affect approximately 45,000 patients annually. Approximately 10,000 men and women die from this disease each year. Bladder tumors may be categorized as superficial non muscle-invading lesions and deep tumors with muscle invasion. Spread of the disease in more advanced stages may distort the tissues surrounding the bladder neck or urethra due to pressure from adjacent tumor or due to actual tumor invasion. Evaluation of the periurethral tissues in a bladder cancer patient may assist in identifying the stage of the disease. While endoscopic resection is suitable for superficial well-differentiated lesions, invasive tumors may require cystectomy with supravesical urinary diversion. Evaluating the degree of involvement of the bladder wall may assist the clinician with selecting the appropriate therapeutic modality.
Cancers of the female genital tract may, under certain circumstances, involve periurethral tissues. For example, in carcinoma of the cervix, spread occurs through lymphatic channels, first extending to the lymph nodes located in the tissues immediately lateral to the cervix. Direct vaginal extension may also occur. Lesions involving these tissues are designated stage two or stage three, depending on the extent of and location of tissues that are involved. Such cancers may be considered locally advanced. For locally advanced cervical cancers, radiation therapy is commonly used rather than excisional therapy. To determine the proper course of treatment, therefore, accurate anatomic information is necessary. Evaluation of periurethral pelvic tissues may provide a such useful information. Similarly, the staging of vulvar or endometrial cancers may benefit from an appraisal of the periurethral anatomy.
Adenocarcinoma of the prostate is diagnosed in about 400,000 patients annually, with approximately 45,000 patients dying from each year. The malignancy may occur in any portion of the gland, with multiple areas of involvement being present about one-third of the time. While early diagnosis is a key to survival, identifying the stage of the disease is crucial to determining proper therapy. The American System of clinical staging is divided into stages A, B, C & D. Stage A, reserved for patients who have no palpable abnormality of the prostate, is divided into A1 and A2 depending on the volume of cancer (A1<5%, A2>5%) removed by transurethral resection (TURP) for presumed benign disease. Stage B represents nodular disease palpated on rectal examination and is divided into B1 disease (nodule 1.5 cm or less) and B2 (nodule>1.5 cm). Stage C is reserved for those patients in whom disease is felt to extend outside the prostate while Stage D for those with metastatic disease. Gradually, the TNM system has gained acceptance especially since it allows for staging of those patients found to have carcinoma as a result of PSA (prostatic specific antigen) elevation and not palpable disease. Stage T1c includes those patients found to have carcinoma based on PSA elevation only, while T1a corresponds to A1, T1b to A2, T2a to B1, T2b to B2, and T3 to C.
Prostate specific antigen (PSA) levels have been correlated with the clinical extent of the disease, but evaluation of the anatomic extent of the disease contributes to determining the appropriate type of surgical intervention. Early-stage carcinoma is understood to be localized to the prostate gland without extension beyond the gland capsule and without extension to distant sites. A spectrum of treatment options exists for prostate cancer, depending on patient age, health, and tumor status. Available options include “watchful waiting”, radiation therapy, radical prostatectomy, cryosurgery and hormonal ablation. Older patients (over 70 years) with small, early cancers may be candidates for watchful waiting. These patients are monitored for development of bladder outlet obstruction symptoms and distal disease. Under some circumstances, this option may also be appropriate for younger patients. Removal of the entire prostate and seminal vesicles, termed a radical prostatectomy, may be performed for organ confined disease, T1 or T2, using either a retropubic or a perineal approach. Radiation therapy provides an alternative to surgery in these patients. Radiation may be delivered either via external beam radiotherapy or via local placement of radioactive seeds within the prostate. Debate exists about the comparative efficacy of radiation therapy vs. prostatectomy. While radiation therapy may involve complications such as diarrhea, cystitis, impotence and incontinence, these last two complications are less frequently encountered with radiation than with surgery. Cryosurgical ablation is another approach to the treatment of organ-confined prostate cancer. To perform this treatment, the physician places stainless steel probes percutaneously into the substance of the prostate under ultrasound guidance. Liquid nitrogen is then circulated through the probes, creating an expanding “iceball” throughout the prostate whose extent can be monitored by using transrectal ultrasound to watch watching a freeze “front” progress within the tissue. The cycle of freezing and thawing of the prostate tissue results in coagulation necrosis of the tissue. As with other forms of therapy, patients treated with cryosurgery are at risk of developing impotence and incontinence after treatment. However, the procedure is minimally invasive and results to date have been encouraging. Advanced carcinoma, by contrast, is not appropriately treated using any of these regimens. It is therefore important to distinguish between those carcinomas confined to the gland and hence adequately treated by radical prostatectomy or other local therapies and those carcinomas extending beyond the gland which are not amenable to local cure. Because the proximal urethra is situated within the prostate gland, anatomic evaluation of tissues surrounding the urethra at the prostatic level may also serve to evaluate the prostate and its malignancies.
Of the non-malignant conditions affecting the urethra and periurethral tissues, urinary incontinence is of particular importance. At least 10 million adults in the U.S. suffer from urinary incontinence, including between 15 and 30 percent of older Americans and at least one-half of the estimated 1.5 million nursing home residents. In addition to the significant psychosocial burden borne by individuals with this ailment, incontinence involves quantifiable health care and related costs, conservatively estimated at approximately $10 billion annually. Urinary incontinence is a symptom, not a disease in itself. It occurs in several clinical patterns, each having a set of possible etiologies.
In many cases, urinary incontinence is a chronic problem, lasting indefinitely unless properly diagnosed and treated. The number of patients with urinary incontinence who have not been successfully treated remains surprisingly high. In part, this is due to inadequate knowledge about the anotomical basis, pathophysiology and potential treatments for the condition. Although new diagnostic tests have been developed, guidelines still remain to be formulated for their appropriate application. Furthermore, although a variety of therapies have been proposed, opinions differ about the best type of treatment to apply to a particular condition.
Evaluation of incontinence presently relies upon routine diagnostic tools such as history and physical examination, combined with specialized studies such as cystometrogram, electrophysiologic sphincter testing, bladder and renal ultrasound, cystourethroscopy, uroflowmetry, and videourodynamic evaluation. Ultrasound has been used transurethrally to evaluate the anatomy and function of the rhabdosphincter in the male urethra. Transvaginal ultrasound and intraurethral ultrasound have been employed in female patients also, to evaluate urinary incontinence.
The advent of intracavity magnetic resonance imaging (MRI) receiver coils for high-resolution clinical imaging of the prostate and of the uterine cervix has shown some promise as a technique for imaging the pelvic floor with increased spatial resolution compared to images acquired with the MRI body coil alone. Driven by a motivation to further increase the signal-to-noise (SNR) ratio at the region of interest, authors have reported the value of these intracavitary coils in the detailed demonstration of the female pelvic anatomy and abnormalities using a transrectal imaging approach as well as transvaginal approach for imaging. External MRI of the urethra and pelvic floor has been carried out in female volunteers and patients to elucidate the relevant regional anatomy. Endorectal and external MRI investigation has also been carried out to evaluate abnormalities of the urethral and periurethral tissues. Despite these efforts, clinical and radiological evaluation of these areas remains difficult and not completely satisfying. Although high-resolution magnetic resonance imaging with phased array pelvic, endorectal and endovaginal coils has dramatically enhanced the ability to visualize abnormalities of the female urethral and periurethral tissues, discussion and controversy still continues about the anatomy of this region.
A number of references disclose systems and methods for evaluating the anatomy of pelvic tissues including the prostate, for example U.S. Pat. Nos. 4,932,411; 5,050,607; 5,170,789; 5,307,814; 5,340,010; 5,355,087; 5,365,928; 5,451,232; 5,413,104; 5,476,095. However, there remains a need in the art for detailed and satisfactory anatomic information about the urethra and pelvic floor, especially in the female. This information would provide a significant contribution to the understanding of urinary incontinence and other urethral abnormalities in female patients, thereby contributing to an understanding of surgical approach for treatment. There further remains a need in the art for methods to provide detailed anatomic information about the male urethra and prostate in order to guide current therapeutic techniques and in order to permit the development of anatomically more refined approaches to the treatment of prostate cancer.
In addition, there is a need in the art for techniques adaptable to evaluating the anatomy of the pediatric pelvis. Genitourinary abnormalities, whether congenital or acquired, may benefit from precise anatomic diagnosis. For example, extrophy of the urinary bladder is part of a spectrum of anatomic deformities which result from lack of ingrowth of the abdominal mesoderm into the cloacal membrane, an embryologic structure which is part of the infraumbilical covering of the developing abdominal wall. In its complete form the bladder mucosa is exposed on the anterior abdominal wall, the pubic symphysis has not fused and the urethra is epispadic. A lesser degree of the same complex includes epispadius alone. Cloacal extrophy is an extremely rare congenital anomaly in which there are two extrophied hemibladders separated by a bulging cecum and ileal orifice. Correction of the anatomic abnormalities with restoration of normal bladder function and urinary continence as well as normal penile appearance in the male is the goal of treatment. Repair of cloacal extrophy is challenging for it requires staged reconstruction of the pelvic urinary structures as well as the gastrointestinal tract. A more thorough understanding of the complex pelvic and urological anatomy may assist surgeons in planning and executing the necessary reconstructive procedures.
It is understood that diagnostic techniques may be combined with therapeutic techniques for pelvic conditions. A better understanding of regional and local anatomy and an appreciation of the relevant pathology may facilitate accurate and effective treatment. There remains a need in the art for diagnostic techniques that are combinable with therapeutic modalities useful for malignant and nonmalignant conditions in the pelvic region.
Techniques of magnetic resonance imaging (MRI) or magnetic resonance spectroscopy (MRS), specialized radiofrequency (RF) may be applicable to these diagnostic and therapeutic problems. RF receiver coils may be placed at the region of interest to increase the signal-to-noise ratio (SNR) for better image (spectrum) quality in MRI or MRS. RF Receiver coils can broadly be separated into categories of volume coils, surface coils and endoluminal coils. Volume coils contain the region of interest within their volume and the imaging region is directed toward the inside of the coil. Surface coils are placed on top of the region of interest and their imaging region is directed to either side of the coil. Endoluminal coils are inserted into natural (urethra, prostate, vagina, rectum, oesophagus, pancreas, etc.) or artificial (endovascular, etc.) orifices of the human or animal body. Their imaging region is directed towards the outside of the coil to provide high-resolution imaging (spectra) of the region surrounding the coil. Different coil designs for a potential endoluminal application are available in the art. The endoluminal RF coils designs exist in multiple forms, including: rigid (GE prostate biopsy guidance coil, (R. D. Watkins, K. W. Rohling, E. E. Uzgiris, C. L. Dumoulin, R. D. Darrow and R. O. Giaquinto, “Magnetic Resonance Image Guided Biopsy in Prostate”, page 412, Book of Abstracts, ISMRM 2000), flexible (Atalar, E., P. A. Bottomley, and E. A. Zerhouni, Method of Internal Magnetic Resonance Imaging and Spectroscopic Analysis and Associated Apparatus, Assignee: Johns Hopkins University: U.S. Pat. No. 5,699,801, Dec. 23, 1997), and others. A quadrature/phased array endo-luminal design was also disclosed earlier (Atalar, U.S. Pat. No. 5,699,801).
However, there remains a need for providing high SNR and increased signal homogeneity in endocavitary designs. Mutual inductance between two or more independent but geometrically adjacent coils, tuned to the same resonance frequency may under certain circumstances improve the SNR and signal homogeneity, but such designs can also result in coupling between the coils that might result in poor signal performance and decreased signal penetration depth into the tissue. Usually, this mutual inductance is compensated for by adding combinations of capacitors, inductors, and/or other electronic elements to the resonant circuit of the coils. Another means of compensation is, to mechanically align the two or more coils in a way to each other, so that the coils are geometrically isolated from another.
Geometric decoupling, however, does not always result in sufficient isolation. The overall coil performance might be still degraded due to residual coupling between the coils. However, there has been no simple method of eliminating coupling between elements of these types of designs taught, thereby impeding the clinical uses of such devices. The use of a metallic paddle to steer the magnetic flux of a coil as a means to insulate two or more adjacent coils from each other was published in the literature in 1946 by Bloch, Hansen and Packard (F. Bloch, W. W. Hansen, M. E. Packard, Phys. Rev. 70:474 (1946)) and was then reconsidered by Andrew (E. R. Andrew, Nuclear Magnetic Resonance. Pp.56-63, Cambridge Univ. Press, London, (1955)) and then by Hoult et al. (D. I. Hoult, C. N. Chen, V. J. Sank, Quadrature detection in the laboratory frame. Magn. Reson. Med. 1, 339-353 (1984)). In these publications, however, a rather small paddle was used to decouple large volume coils from one another, to minimize distortions of the B1 field homogeneity to the inside of the coil. The application of such a paddle to an ‘inside out’ design of endoluminal coils, as first described in this report, enabled the paddle to be inserted into the most sensitive region between the coils. Furthermore, the paddle could be designed larger relative to the size of the coils, providing a very effective means of coil insulation, virtually without affecting the B1 field toward the outside of the coil. No additional electronics were required to achieve an isolation of about 50 dB. Signal homogeneity as well as signal penetration depth and therefore image quality was markedly improved by minimizing the mutual inductance between the coils. There remains a need in the art, therefore for a means to minimize the mutual indictance between two or more independent RF transmit or receive coils in MRI or MRS that is simple to implement and effective. There is a further need to device a RF coil for endoluminal applications that can improve signal homogeneity and signal penetration depth.